TRANSACTIONS OF SECTION D. 683 



the action of the other organs of the body. If we were to alter the size or shape 

 of some part of a simple machine it might be able still to do its work the better 

 or the worse for the change, but if we were to alter the corresponding part of a 

 complicated machine it would probably throw it out of gear and prevent any work 

 being done at all. 



From this consideration we gather that in the process of the evolution of the 

 higher forms of life there has been a gradual diminution in the range of variation 

 of the different characters of the body, a gradual diminution of the response of 

 these characters to changes of the environment. Characters which, in the early 

 stages of evolution, were probably plastic become rigid. 



The gradual evolution of the power of co-ordinated movement has been un- 

 doubtedly accompanied by a loss in the variability of the shape of the body, the 

 gradual evolution of a blood vascular system and nervous system has led to a loss 

 of variability in the alimentarj^ canal with which they are associated. In the 

 majority of cases, however, we are much too ignorant of the facts of the co-ordi- 

 nation of the parts of the body or of the co-ordination of any one part to the 

 environment to be able to frame an hypothesis as to why any one character has be- 

 come rigid. It is difficult to see the reason why the number of the tentacles and 

 mesenteries in Alcyonian polyps has become fixed at eight, while in other Coeleu- 

 terates these characters are so variable, or why the colour of Tubipora is always 

 red, and of Melitodes variable. 



The study of species, however, teaches us that, in all cases, except perhaps 

 in some examples of degeneration, the plastic condition of the characters was 

 antecedent to the rigid, that in the earlier stages of evolution the conditions of 

 extreme plasticity and ready response to changing external conditions were neces- 

 sary for the survival of the species ; and that in the later stages, when special 

 adaptations to special circumstances were developed, a certain rigidity or indifler- 

 ence to changing external conditions was equally necessary for its survival. 



Now, the stud)^ of the various orders of Crelenterates conveys a very strong 

 impression that the part played by the environment in the production of the 

 variations of the adult is much greater in proportion than it is in the higher groups 

 of animals. It is true that direct proof of this is wanting. Such a direct proof 

 can only be obtained by experiments in rearing and breeding under varying 

 conditions, and there are at present many serious diihculties to overcome before 

 experiments of this nature can be satisfactorily made. 



Nevertheless, the circumstantial evidence in favour of the truth of this impres- 

 sion is, to my mind, so strong that we are justified in considering its bearing upon 

 the general question. It is quite impossible for me on this occasion to set before 

 you at all adequately the general nature of this circumstantial evidence. To do 

 so would involve statements concerning the actual variations of a Large number of 

 species already observed in one locality and in several widely distributed localities, 

 with a discussion of the possible direct influence of the conditions of such localities, 

 so far as they are known, upon each of the principal variations. Such statements 

 would necessarily be of such a special and technical kind that, even if time per- 

 mitted me to make them, they would not be suitable for an Address of this character. 

 I may be permitted to say, however, that I am collecting and preparing the 

 evidence for publication on this point at a later date. There can be no doubt, 

 however, from the evidence 1 have already submitted to you in part, that some 

 species are far more influenced by changes in the environment, or, to simplify the 

 expression, are far more plastic than others ; and we may conclude that in the 

 evolution of other groups of animals the earlier forms were far more plastic than 

 their modern descendants. In the earlier stages of evolution there must have 

 been in the first instance a lessening of the power of change in structure according 

 to change of environment. The fixity or rigidity of certain characters thus pro- 

 duced enabled a more elaborate co-ordination both in form and action to occur 

 between one set of organs and another. It permitted a further localisation and 

 specialisation of functions, or, in other words, further differentiation of the animal 

 tissues. 



Accompanying this differentiation there was a loss in the power of regeneration. 



